1. Field of the Invention
The present invention relates to the fabrication process of thin film semiconductor devices suitable for active matrix liquid crystal display devices, and the like.
2. Description of the Related Art
Previously, a fabrication process such as that described below has been adopted for the low temperature production of polycrystalline silicon thin film transistors (p-Si TFT), representative of thin film semiconductor devices, at temperatures of 600xc2x0 C. or below at which conventional glass substrates can be used, or for the low temperature production of amorphous silicon thin film transistors (a-Si TFT) in a similar temperature range of 425xc2x0 C. or less. First, an amorphous silicon layer which will serve as the semiconductor film is deposited on a substrate to a thickness of approximately 50 nm by low pressure chemical vapor deposition (LPCVD). Next, the amorphous silicon layer is irradiated by a XeCl excimer laser (wavelength of 308 nm) to produce a polycrystalline silicon layer (p-Si layer). Because the absorption coefficients for the XeCl laser light for amorphous silicon and polycrystalline silicon are large at 0.139 nmxe2x88x921 and 0.149 nmxe2x88x921 respectively, 90% of the laser light incident upon the semiconductor film is absorbed within the first 15 nm. Additionally, the absorption coefficient for amorphous silicon is 7% less than the absorption coefficient for polycrystalline silicon. Following the laser irradiation step, a silicon oxide layer which serves as the gate insulator layer is deposited by either chemical vapor deposition (CVD) or physical vapor deposition (PVD). Next, a gate electrode of tantalum or other material is formed to create a metal (gate electrode)-oxide (gate insulator layer)-semiconductor (polycrystalline silicon layer) field effect transistor (MOS-FET) structure. Finally, an interlevel dielectric layer is deposited on top of these layers; and following the opening of contact holes, interconnects are created from metal films, and the thin film semiconductor device is complete.
During this fabrication procedure for thin film semiconductor devices, however, it is difficult to control the energy density of the excimer laser; and even a slight variation in energy density can lead to a large degree of variation in the quality of the semiconductor film within the same substrate. Further, if the energy density is even slightly more than a threshold value determined by such factors as the film thickness and hydrogen content of the film, considerable damage can be inflicted on the semiconductor film and lead to significant degradation in the semiconductor characteristics and production yield. As a result of these factors, it is necessary to set the laser energy density considerably less than the optimal value in order to obtain a uniform polycrystalline semiconductor film over the entire substrate; and the energy density is insufficient to produce good quality polycrystalline thin films. Additionally, even if the optimal energy density is used for irradiation, it is difficult to increase the size of the grains comprising the polycrystalline film; and it is known that many defects remain within the film. Consequently, it is not possible to reliably produce thin film semiconductor devices such as p-Si TFTs using the technology of the prior art without sacrificing the electrical characteristics of the completed thin film semiconductor devices.
Therefore, in consideration of the all the issues described above, the objective of the present invention is to provide a fabrication procedure to reliably produce superior thin film semiconductor devices with a low temperature process which is 600xc2x0 C. or less, and ideally 425xc2x0 C. or less.
The present invention is a fabrication process of a thin film semiconductor device having a crystalline semiconductor film formed on a substrate, said semiconductor film being an active layer of a transistor and being mainly composed of silicon (Si), the process comprising: an underlevel protection layer formation step of forming a silicon oxide film as an underlevel protection layer on the substrate; a first processing step of forming a semiconductor film, which is mainly composed of silicon (Si), on the underlevel protection layer; and a second processing step of irradiating a pulsed laser light on the semiconductor film, wherein the absorption coefficient of said pulsed laser light is larger in amorphous silicon than in polycrystalline silicon.
The present invention is further characterized by the fact that the absorption coefficient of the pulsed laser light in polycrystalline silicon mpSsi is 10xe2x88x923 nmxe2x88x921 or higher and 10xe2x88x922 nmxe2x88x921 or lower. In this case, if the film thickness of the semiconductor film is taken to be d, it is desirable for the film thickness d and the previously described absorption coefficient mpSi to satisfy the following relationship
0.105xc2x7mpSixe2x88x921 less than d less than 0.693xc2x7mpSixe2x88x921.
Ideally, the following relationship is satisfied
0.405xc2x7mpSixe2x88x921 less than d less than 0.693xc2x7mpSixe2x88x921.
In order for the present invention to be applicable to liquid crystal display devices, it is desirable for the substrate to be transparent to visible light. Further, regardless of the application, its also desirable for the substrate to be essentially transparent to the pulsed laser light. xe2x80x9cEssentially transparentxe2x80x9d means that the absorption coefficient of the pulsed laser light in the substrate is at least one tenth the absorption coefficient in polycrystalline silicon or lower. Specifically, the absorption coefficient of the substrate mSub should be approximately 10xe2x88x924 nmxe2x88x921 or lower. Normally, formation of the semiconductor film mentioned above would include a deposition step by chemical vapor deposition (CVD). Within the chemical vapor deposition process category, low pressure chemical vapor deposition (LPCVD) is particularly applicable; and it can be said further that semiconductor film deposition in a high vacuum low pressure chemical vapor deposition chamber is ideal. A high vacuum low pressure chemical vapor deposition chamber is one in which the background pressure just prior to semiconductor film deposition is typically 5xc3x9710xe2x88x927 Torr or less. It is desirable for the pulsed laser light to be produced by a solid state light emitting element, and a pulsed laser light produced by the second harmonic of a pulsed Nd: YAG laser (abbreviated as YAG 2w) is the best. When a YAG 2w laser light impinges on a semiconductor film comprised mainly of silicon, it is preferable for the thickness of the semiconductor film to be approximately 25 nm or more and approximately 165 nm or less and, ideally, to be approximately 25 nm or more and approximately 95 nm or less.
During laser irradiation in the second process step, the pulsed laser light irradiation region on top of the semiconductor film has a width W (mm) and a length L (mm) and ranges from a line profile to an approximately rectangular profile. Within the irradiation region, the pulsed laser""s irradiation energy density has a roughly trapezoidal distribution along the length of the region. On the other hand, along the width of the region, it is preferable to have a irradiation energy distribution which is either trapezoidal or Gaussian. It is desirable for the ratio (L/W) of the irradiation region length, L, to the width, W, to be greater than or equal to 100, and ideally greater than or equal to 1000. It is preferable for the maximum gradient of the irradiation energy density along the width of the profile to have a value of 3 mJxc2x7cmxe2x88x922xc2x7mmxe2x88x921 or higher. The entire substrate is laser irradiated by translating the irradiation region having the features described above along the width of the profile after each irradiation. A second process step in which a given point on the semiconductor film is covered by more than approximately 10 and less than approximately 80 pulsed laser irradiations during this process is desirable.